The advent of more stringent environmental regulations together with the ever increasing demand for high octane gasoline, has prompted refiners to seek energy efficient methods for maximizing gasoline octane and yield. Process integration, or coupling of chemical conversion processes, has gained wide acceptance in the refining industry as a means for improving product volume and quality while decreasing energy consumption. The benefits, however, are not without drawbacks, which include loss of process unit flexibility if two or more conversion processes must be operated concurrently or not at all.
Processes for upgrading heavier petroleum crude fractions to more saleable distillate and gasoline fractions include catalytic and thermal cracking. These processes convert a gas oil or heavier cut from a crude feedstock to the desired distillate and gasoline fractions, but a portion of the crude feedstock is also cracked to lighter fractions such as C.sub.4 -aliphatics, which are less valuable than the heavier distillate and gasoline products.
Catalytic cracking processes manufacture a major segment of the total gasoline pool produced in modern oil refineries by upgrading gas oil and heavier feedstreams to a lighter product slate including gasoline and distillate as well as C.sub.4 -aliphatics rich in olefins. Examples of such catalytic cracking processes are described in P. B. Venuto and E. T. Habib, Jr., Fluid Catalytic Cracking with Zeolite Catalysts (1979) as well as U.S. Pat. Nos. 2,383,686 to Wurth, 2,689,210 to Leffer, 4,093,537 to Gross et al., 4,118,338 to Gross et al., and 4,411,773 to Gross, which patents are incorporated by reference herein.
To increase the overall yield of high octane gasoline from catalytic cracking units, processes have been developed which upgrade the C.sub.4 -byproducts of the cracking process. With the advent of these light aliphatics upgrading processes, the demands on the catalytic cracking unit product fractionation section have also changed. Specifically, the C.sub.4 -aliphatics upgrading processes operate at relatively high temperature conditions, typically above about 800.degree. F. For this reason, the H.sub.2, H.sub.2 S, and mercaptan sulfur contents of the C.sub.4 -aliphatic streams from the catalytic cracking unit product fractionation section are critical, not only to meet product specifications and to prevent accelerated catalyst deactivation, but also to assure safe and reliable unit operation using the most economical materials of construction. It has been found that levels of H.sub.2 S, H.sub.2, and mercaptan sulfur levels which were completely acceptable for lower temperature light aliphatics upgrading processes such as HF or H.sub.2 SO.sub.4 catalytzed alkylation can markedly accelerate corrosion, pitting and cracking in carbon steel and lower alloy vessels under the more severe temperature conditions associated with the catalytic upgrading processes presently under consideration. Thus it would be desirable to provide the light aliphatics upgrading process associated with the catalytic cracking unit with a C.sub.2 -C.sub.4 aliphatic stream which is relatively free from H.sub.2 S, H.sub.2, and mercaptan sulfur.
Catalytic cracking process units typically include a main fractionator, commonly called the main column, which receives cooled reactor effluent from the catalytic cracking process. The main column fractionates this reactor effluent into a plurality of streams including clarified slurry oil, heavy cycle oil, light cycle oil, light and heavy distillates, gasoline and an overhead gas stream rich in C.sub.4 -olefins. The gasoline and lighter components are then further fractionated in an unsaturated gas plant which typically includes, in order, a deethanizer absorber, a debutanizer and a depropanizer.
The deethanizer absorber splits the gasoline and lighter material into a C.sub.2 -overhead gas stream and a C.sub.3 +bottoms stream. The C.sub.2 -overhead gas stream may optionally be treated in a sponge absorber to further sorb C.sub.3 +components before acidic components such as hydrogen sulfide, carbon dioxide and hydrogen cyanide are removed in a purification sorption column. Having been treated to reduce its acidic gas content, the deethanizer absorber overhead stream is then charged to a fuel gas header to be burned for fuel in the refinery complex.
The deethanizer absorber bottom stream is then charged to a debutanizer fractionator where it is split into a C.sub.5 +gasoline stream rich in olefinic components and a C.sub.3 -C.sub.4 overhead stream. The debutanizer fractionator is typically designed to meet a bottom stream gasoline volatility specification requiring vapor pressure of less than about 10 psi. Finally, the debutanizer overhead stream, rich in C.sub.3 -C.sub.4 olefins, may be fractionated into a propane/propylene overhead stream and a butane/butylene bottoms stream. This step is most often employed when additional light aliphatics upgrading capacity is available, for example, an alkylation process unit for converting iso- and normal C.sub.4 -aliphatics to high octane alkylate gasoline. The C.sub.3 -rich depropanizer or debutanizer overhead stream may be sold as LPG, but first must be treated in a mercaptan sulfur removal process to meet sulfur content specifications. One example of such a process is the Merox process (trademark and/or service mark of UOP, Inc.).
The incremental volume of C.sub.2 -fuel gas generated by a catalytic cracking process may increase the total refinery fuel gas volume beyond that needed to fulfill its fuel gas consumption and sales requirements. To assure compliance with environmental regulations governing content and volume of gases exhausted to the atmosphere, fuel gas production is limited to the total volume which can be consumed within the refinery, sold to consumers beyond the battery limits of the refinery, or flared in accordance with the applicable environmental permits. Thus if the incremental volume of fuel gas generated by the catalytic cracking unit exceeds the capacity of facilities for its disposition, the cracking unit feedrate or reaction severity must be reduced. Neither option is economically desirable. The ideal solution would be to decrease fuel gas volume by shifting the overall yield from the catalytic cracking unit away from C.sub.2 -components and toward more valuable high octane C.sub.5 +gasoline. The acid gas components of the catalytic cracking unit reactor effluent stream tend, however, to be carried with ethane and ethylene. This is a major obstacle to successfully integrating the aliphatics upgrading process with the catalytic cracking process. Clearly, then, the problem of excess fuel gas production cannot be solved merely by shifting the cut points in a conventional catalytic cracking product fractionation section because the downstream light aliphatics upgrading process would be exposed to hydrogen and acid gases under severe temperature conditions.
A number of acid gas removal processes are commercially available for treating this overhead stream including chemical solvent as well as physical sorption processes. Chemical solvent techniques include countercurrent contacting with monoethanolamine (MEA), diethanolamine (DEA) and hot potassium carbonate. Physical sorption techniques employ solid sorbents such as molecular sieves, actiated charcoal and iron sponge.
Conventionally, these acid gas removal processes are installed downstream of the sponge absorber and debutanizer. Consequently, the acid gases are carried through the various upstream separation processes of the USGP including the absorber-deethanizer, sponge absorber and debutanizer. This configuration tends to increase the rate of acid gas induced corrosion of a large portion of the vessels and ancillary equipment in the USGP, leading to increased maintenance operations and plant downtime. Under the more severe temperature conditions of catalytic aliphatics upgrading processes, streams containing these acidic components readily attack carbon steel and the lower chromium- and molybdenum-containing steel alloys, and may cause cracking, pitting, blistering, or general thinning.
Catalytic aromatization converts the light aliphatics over a catalyst, for example a medium-pore zeolite catalyst such as ZSM-5, to a product mixture rich in aromatics. Oligomerization and olefin interconversion may employ similar catalysts, but are typically conducted under less severe temperature conditions.
Thus it is clear that a process for shifting product yield in a catalytic cracking unit away from C.sub.4 -light aliphatics, particularly C.sub.2 -fuel gas, to favor production of high octane gasoline would provide substantial operational and economic benefits. Further, it would be desirable to provide the light olefin upgrading section of such a process with a feedstock of sufficient purity to meet the application environmental standards and product quality specifications while also avoiding the incremental capital costs associated with alloyed process equipment. Still further, it would be beneficial to segregate the various processing steps according to their respective operating pressure requirements to avoid energy losses due to unnecessary pressure drops in the catalytic cracking process product fractionation section.